The present invention relates generally to an active-hydrogen-containing phosphorus compound for cross-linking a resin and for imparting flame-retardancy to the cured resin, and in particular to a cured frame-retardant epoxy resin prepared by reacting the hardener with a di- or poly-functional epoxy resin via an addition reaction between the active hydrogen and the epoxide group.
Epoxy resins have the excellent characteristics of moisture, solvent and chemical resistance, toughness, low shrinkage on cure, superior electrical and mechanical resistance properties, and good adhesion to many substrates. The versatility in formulation also make epoxy resins widely applicable industrially for surface coatings, adhesive, painting materials, potting, composites, laminates, encapsulants for semiconductors, and insulating materials for electric devices, etc. o-Cresol formaldehyde novolac epoxy (CNE) is the resin typically employed in the encapsulation of microelectronic devices. Several approaches for modification of epoxy backbone for enhancing the thermal properties of epoxy resins have been reported. Aromatic bromine compounds in conjunction with antimony oxide are widely used as a flame retardant for epoxy resins. Tetrabromobisphenol A is a typical example of the aromatic bromine compounds used as a flame retardant for epoxy resins. An excess amount of epoxy resin is reacted with tetrabromobisphenol A to prepare an advanced epoxy resin having two terminal epoxide groups, as shown in the following formula: 
wherein Ep is a bi-radical group of the backbone of the epoxy resin, and m is an integer of 1-10. The advanced epoxy resin can be used in preparing a flame-retardant printed circuit board (FR-4) by impregnating glass fibers with the advanced epoxy resin and heating the resulting composite to cure the advanced epoxy resin. Furthermore, the advanced epoxy resin can be employed to encapsulate microelectronic devices, in which the advanced epoxy resin is cured at a high temperature with a curing agent, so that an encapsulant having a flame-retardant property is formed. Typical examples can be found in U.S. Pat. No. 3,040,495 (1961); U.S. Pat. No. 3,058,946 (1962); U.S. Pat. No. 3,294,742 (1966); U.S. Pat. No. 3,929,908 (1975); U.S. Pat. No. 3,956,403 (1976); U.S. Pat. No. 3,974,235 (1976); U.S. Pat. No. 3,989,531 (1976); U.S. Pat. No. 4,058,507 (1997); U.S. Pat. No. 4,104,257 (1978); U.S. Pat. No. 4,170,711 (1979); and U.S. Pat. No. 4,647,648(1987)].
Although the tetrabromobisphenol A-containing advanced epoxy resin shows flame retardant property, major problems encountered with this system are concerned with the generation of toxic and corrosive fumes during combustion such as dioxin and benzofuran.
The flame retardant having a small molecular weight tends to lower the mechanical properties of the epoxy resins, and migrate/vaporize from the epoxy resins such that the flame retardancy thereof diminishes.
The trend of electronics equipment is being miniaturized and becoming thinner, at the same time the scale of integration of large scale integrated circuits (LSICs) is continuing upward, forcing the design toward larger chips, finer patterns, and higher pin counts that are more susceptible to a high-temperature failure. The prevailing surface mount technology (SMT) also causes the devices being subjected to a high temperature. Therefore, the development of a high-temperature reliable, flame-retardant and environmentally friendly epoxy resin for printed circuit board and encapsulant are essential for semiconductor industry.
It is an object of this invention to provide a phosphorus-containing flame retardant hardener for cross-linking a resin and for imparting flame-retardancy to the cured resin.
It is another object of this invention to provide cured epoxy resins with good thermal stability, superior heat resistance, and without environmental problem, which are suitable for use in making printed circuit boards and in semiconductor encapsulation applications.
In order to accomplish the aforesaid objects, a flame-retardant hardener containing one of the following phosphorus groups was synthesized in the prevent invention: 
wherein Ar is an un-substituted or substituted phenyl or phenoxy radical. The hardener of the present invention is prepared by bounding the phosphorus-containing rigid group to a multi-active-hydrogen-containing compound or resin.
The present invention also provides a cured flame-retardant epoxy resin by using the hardener of the present invention. The cured flame-retardant epoxy resin so prepared has a high glass transition temperature (Tg), high decomposition temperature and high elastic modulus, and is free of toxic and corrosive fumes during combustion, and thus is suitable for printed circuit board and semiconductor encapsulation applications.
A phosphorus-containing hardener prepared in accordance with the present invention has a formula selecting from the group consisting of (a), (b), (c) and (d): 
wherein
m=1 or 2; mxe2x80x2=0 or 1; p=0xcx9c3; R=C1xcx9cC4 alkyl or aryl; X=O, S or NH; 
wherein Q=xe2x80x94, when Qxe2x80x2 is the latter; 
wherein
R1, R2 independently are H, C1xcx9cC18 alkyl, C6xcx9cC18 aryl, C6xcx9cC18 substituted aryl, C6xcx9cC18 aryl methylene, or C6xcx9cC18 substituted aryl methylene;
nxe2x80x2=0xcx9c11; Z=xe2x80x94NH2, xe2x80x94NHR, or xe2x80x94R; o=1xcx9c3; oxe2x80x2=3xcx9c10; r=0xcx9c6; R, Q and p are defined as above; 
wherein R is defined as above and n=0-5;
wherein either all the A or all the Axe2x80x2 in each formula of (a) to (d) are H, and at least one of the A is not H when all the Axe2x80x2 are H in each formula of (a) to (d), and at least one of the Axe2x80x2 is not H when all the A are H in each formula of (a) to (d).
Preferably, R is hydrogen or methyl, and more preferably R is hydrogen.
Preferably, 
Preferably, X is xe2x80x94Oxe2x80x94 or xe2x80x94NHxe2x80x94. More preferably, X is xe2x80x94Oxe2x80x94.
Preferably, Y isxe2x80x94, i.e. r is 0.
Preferably, the hardener of the present invention has a structure of the formula (a).
Preferably, the hardener of the present invention has a structure of the formula (b).
Preferably, the hardener of the present invention has a structure of the formula (c).
Preferably, the hardener of the present invention has a structure of the formula (d).
Preferably, all the Axe2x80x2 are H, and 
More preferably, only one A is not H.
Preferably, all the A are H, and only one Axe2x80x2 is not H.
Preferably, all the A are H, and 
Preferably, all the A are H, and Qxe2x80x2 is 
More preferably, Z is xe2x80x94NH2.
Preferably, R1xe2x80x94Cxe2x80x94R2 is one of the followings: 
wherein Xxe2x80x2=H or halogen. More preferably, R1 and R2 are hydrogen.
The hardener of the present invention can be synthesized by bounding a reactive phosphorus-containing rigid group to a multi-active-hydrogen-containing compound or resin. There are two different schemes for preparing the hardener of the present invention depending on the types of the reactants containing the reactive phosphorus-containing rigid group. The reactants having the following formulas (I) or (II) are used to prepare the hardener having all the A in the formulas (a) to (d) being hydrogen: 
by reacting with a multi-active-hydrogen-containing compound or resin having a structure selected from the formulas (III) to (VII) as follows: 
wherein R1, R2, Ar, R, Qxe2x80x2, X, Z, Y, p, o and oxe2x80x2 in (I) to (VII) are defined the same as above.
The reactants having the following formulas (Ixe2x80x2) or (IIxe2x80x2) are used to prepare the hardener having all the A in the formulas (a) to (d) being hydrogen: 
by reacting with a multi-active-hydrogen-containing compound or resin having a structure selected from the formulas (III), (V), (VI) and (VII), wherein Ar in the formula (IIxe2x80x2) is defined as above.
The compound (I) may be synthesized by reacting 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) with a compound of R1CR2O, as shown by the following reaction formula (VIII): 
The compound (II) may be synthesized by carrying out a reaction as shown by the following reaction formula (IX): 
R1, R2, and Ar in the formulas (VII) and (IX) are defined as above.
A reaction suitable for synthesizing the phosphorus-containing halide, 2-(6-oxid-6H-dibenz less than c,e greater than  less than 1,2 greater than oxa-phosphorin-6-yl) chloride [ODOPC; (Ixe2x80x2)], is shown as follows (X): 
A reaction suitable for synthesizing the phosphorus-containing halide (IIxe2x80x2), is shown as follows (XI): 
wherein R, n and Ar are defined as above.
The present invention further synthesized a phosphorus-containing flame-retardant cured epoxy resin by curing an epoxy resin or advanced epoxy resin with the hardener of the present invention alone or together with a curing agent for an epoxy resin in a molten state. The curing agent can be any curing agent used in the art for curing an epoxy resin, and preferably is selected from the group consisting of phenol-formaldehyde novolac, dicyandiamide, methylenedianiline, diaminodiphenyl sulfone, phthalic anhydride and hexahydrophthalic anhydride. Preferably, the curing reaction is carried out at a temperature higher than 150xc2x0 C. and with a stoichiometric amount of the hardener and the curing agent, i.e. the equivalent ratio of the epoxide group in the epoxy resin and/or advance epoxy resin and the functional groups in the hardener and the curing agent is about 1:1. More preferably, the curing reaction is carried out in the presence of a curing promoter such as triphenylphosphine, and in an amount of 0.01-10.0 parts by weight of the curing promoter per 100 parts by weight of the epoxy resin and/or advance epoxy resin. The phosphorus-containing flame-retardant cured epoxy resin of the present invention is suitable for use in making a flame-retardant printed circuit board as a matrix resin and in semiconductor encapsulation.
A suitable epoxy resin for use in the present invention can be any known epoxy resin, for examples those having two epoxide groups such as bisphenol A epoxy resin, bisphenol F epoxy resin, bisphenol S epoxy resin and biphenol epoxy resin, and those having more than two epoxide groups such as phenol formaldehyde novolac epoxy and cresol formaldehyde novolac epoxy (CNE) with 4-18 functional groups, and mixtures thereof, for examples, those having the formulas (axe2x80x2) to (dxe2x80x2) as follows: 
wherein 0 less than t less than 12; R3=H or C1-C4 hydrocarbon radical; R4 and R5 independently are hydrogen, methyl or 
wherein R3 is defined as above; and 
wherein E and Q are defined as above; 
wherein E and Q are defined as above; and 
wherein E and Y are defined as above.
An advanced epoxy resin suitable for use in the present invention can be prepared by conducting a curing reaction of a conventional curing agent for an epoxy resin and using an excess amount of an epoxy resin in a molten state.
In synthesizing the phosphorus-containing flame-retardant cured epoxy resin, the active hydrogen of the hardener of the present invention, xe2x80x94XH in the formulas (a) to (d), reacts with the epoxide groups of the epoxy resin or advanced epoxy resin. Taking the hardener having a structure of the formula (c) as an example, the curing reaction can be shown as follows: 